Previous: The pits were found in the farm hamlet of La Joya in the township of San Fernando, in the same area where the bodies of 72 migrants, most from Central America, were found murdered last August.

Mexican security forces searching for abducted bus passengers in a violent northern state bordering Texas have stumbled on a collection of pits holding a total of 59 bodies.

The grisly find was made near the ranch where drug cartel gunmen less than a year ago massacred 72 migrants who were trying to reach the United States.

Investigators struggled to exhume the bodies in the mass grave to determine whether they belonged to kidnapped bus passengers, migrants who frequently ride buses in the area, or drug traffickers executed by rivals.

Meanwhile Amnesty International today urged the Mexican authorities to conduct a full, independent and effective investigation into the deaths.

Rupert Knox, researcher on Mexico at Amnesty International, said: ‘The mass graves found yesterday once again show the Mexican government’s failure to deal with the country’s public security crisis and reduce criminal violence which has left many populations vulnerable to attacks, abductions and killings.

‘All too often such human rights crimes have gone unpunished, leaving criminal gangs, and officials acting in collusion with them, free to target vulnerable communities, such as irregular migrants.’

Tamaulipas state investigators and federal authorities went to the site about 80 miles south of the border at Brownsville, Texas, to investigate reports that gunmen had begun stopping buses and pulling off some passengers in the area starting March 25.

Two other such cases were reported in subsequent days, in what may have been an attempt at forced recruitment by a drug gang, Tamaulipas state interior secretary Morelos Canseco said.

The gunmen reportedly abducted almost exclusively men and allowed the remaining passengers to continue on their way.

State and federal investigators and soldiers conducted the raid, but differed on what exactly happened.

The federal Interior Department said the first pit was discovered Saturday and soldiers detained five suspected kidnappers. Tamaulipas officials said the pits were found Wednesday, and a total of 11 suspected kidnappers were captured and five kidnap victims were freed. The reason for the discrepancy was not clear.

But the security forces agreed that a series of eight burial pits had been found, one of which contained 43 bodies and the others 16 corpses. The bodies were being examined to determine their identities and cause of death.

Canseco said two of the dead were women. Many of the victims found in the pits appeared to have died between 10 and 15 days ago, dates that would roughly match the bus abductions, he said.

A statement from the Tamaulipas government, which ‘energetically condemned’ the killings, did not say what drug gang, if any, the suspects belonged to.

President Felipe Calderon’s office issued a statement saying the find ‘underlines the cowardliness and total lack of scruples of the criminal organizations that cause violence in our country.’

While there was no immediate confirmation that a drug cartel was involved, officials refer to the cartels as ‘criminal organizations.’

The pits were found in the farm hamlet of La Joya in the township of San Fernando, in the same area where the bodies of 72 migrants, most from Central America, were found shot to death Aug. 24 at a ranch.

Authorities blamed that massacre on the Zetas drug gang, which is fighting its one-time allies in the Gulf cartel for control of the region.

The victims in the August massacre were illegal immigrants from El Salvador, Honduras, Guatemala, Ecuador and Brazil. An Ecuadorean and Honduran survived the attack, which Mexican authorities say occurred after the migrants refused to work for the cartel.

Mexican drug cartels have taken to recruiting migrants, common criminals and youths, authorities say.

But drug gunmen also operate kidnapping rings, and put up roadblocks on highways in Tamaulipas and other northern states, where they hijack vehicles and rob and sometimes kill passengers.

San Fernando is on a major highway that leads to the U.S. border, but it wasn’t immediately known whether the victims found in the mass grave had been kidnapped from that road.

Drug gangs across Mexico also sometimes use mass graves to dispose of the bodies of executed rivals.

The wave of drug-related killings – which has claimed more than 34,000 lives in the four years since the government launched an offensive against drug cartels – drew thousands of protesters into the streets of Mexico’s capital and several other cities Wednesday in marches against violence.